Circuit board

ABSTRACT

A circuit board includes a plurality of through holes into which a plurality of leads of one electronic devices are inserted and soldered with lead free solder. Among these through holes, the volume of through hole into which the outermost end lead of leads of the electronic device is inserted, is set greater than the volume of through hole, into which the lead at the position nearest to the center of the electronic device is inserted.

TECHNICAL FIELD

The present invention relates to a circuit board, and in particular,relates to a circuit board that includes through holes and that issuitable for mounting an electronic insert mounting device having leadsto be inserted into the through holes and soldered, by means ofsoldering with lead-free solder.

BACKGROUND ART

A lot of leadless electronic devices are surface-mounted on a circuitboard. On the other hand, in regard to some electronic devices, such asa connector and a variable resistor, their leads are inserted andsoldered into through holes as electronic insert mounting devices. Inthis description, the through hole means a hole such as a plated throughhole, a wall surface of which is covered by conductive film.

FIG. 1A is a plan view showing a structure of a portion of theconventional circuit board where electronic insert mounting devices aremounted, and FIG. 1B is a cross-sectional view taken along line A-A inFIG. 1A.

The circuit board that is used to mount electronic devices is usuallymanufactured by undergoing the following steps. First, a prepreg inwhich resin such as epoxy resin and polyimide resin is impregnated intoa glass fabric base material and the resin is partially-hardened, or aprepreg in which phenol resin is impregnated into a paper base materialand the resin is partially-hardened, and a copper-clad laminate in whichcopper foil is affixed on the prepreg by the pressure and heattreatment, are prepared. Then, the copper foil that is affixed on thecopper-clad laminate is patterned by the photo etching method or thelike, and a predetermined number of wiring substrates having copper foilpatterns to be internal layer wires 3 are fabricated. Successively, inorder to improve adhesion to the prepreg, the roughing process(blackening process) is applied to the copper foil surface of the wiringsubstrate. After that, these wiring substrates are laminated through theprepreg so that the wiring circuits are the outermost layers and areintegrated by applying pressure and heat, and then resin laminate 2having internal layer wires 3 inside is manufactured.

Then, holes which will become through holes 4 are opened in resinlaminate 2 by the drilling process. Successively, in order to improvethe connection between internal wires 3 and through holes 4, resin atinternal wires 3 is cleaned (desmeared). After that, the activationprocess, the electroless plating process, and the electrolytic platingprocess are performed to form through holes 4, in each of which aconductive film is formed on a wall surface of the hole. Then, afterthrough holes 4 are protected by the hole plugging process or thetenting process, the outermost copper layer is patterned to formexternal wires 5. At this time, lands 6 are formed around the openingsof through holes 4 in both surfaces of the board. Through holes 4,external wires 5, and lands 6 may be formed by the pattern platingmethod.

Finally, although not shown, solder resists are formed in areas exceptthe solder portions in both surfaces of the board, and then themanufacturing process of multi-layer circuit board 1 is completed.

The above is the manufacturing process of multi-layer circuit board 1having internal wires 3. Alternatively, a double-sided circuit board maybe formed by using a double-sided copper-clad plate as a startingmaterial and by similarly performing the through hole forming step andsubsequent steps.

In FIGS. 1A and 1B, a dashed-dotted line indicated by symbol ◯ shows thecenterline of a case of an electronic device at the time when theelectronic device is mounted. In the example shown in FIGS. 1A and 1B,the electronic device has an elongated plane shape with leads arrangedin a line, and centerline ◯ is the centerline in the longitudinaldirection of the electronic device. In the conventional circuit board,all the shapes and sizes of through holes 4 are equal without making adistinction between center through hole 4 a, into which a lead closestto the center of the case of the electronic device is inserted, andoutermost end through hole 4 b, into which a lead at the outermost endof the case of the electronic device is inserted.

In the process of soldering the electronic device on multi-layer circuitboard 1, which is manufactured like this, generally, after the reflowprocess of mounting surface-mounted devices, such as chip devices andQFP, is performed, the flow process of mounting electronic insertmounting devices is performed.

As solder materials used to solder electronic devices, tin-lead solder,in particular, tin-lead eutectic solder which is close to an eutecticcomposition and which has the mass ratio of Sn and Pb, like Sn:PB=60 to63%:40 to 37%, has been used for a long time. Since tin-lead eutecticsolder is a material having high ductility, during the soldering processor the like, the tin-lead eutectic solder can absorb the stressgenerated by differences of thermal expansions and thermal shrinkagesbetween multi-layer circuit board 1 and the case of the electronicdevice or the like, and the solder can reduce the stress applied tomulti-layer circuit board 1 and the electronic devices.

However, in recent years, due to increasing awareness of environmentalissues, environment pollution caused by lead becomes problematic, and ashift to lead-free solder is rapidly promoted. This lead-free soldermainly includes tin and additionally includes silver, copper, zinc,bismuth, indium, antimony, nickel, germanium, and so on, and hascharacteristics that it is stronger in metal tensile strength and creepstrength and is smaller in ductibility than conventional eutectic solder[typically, Sn is 63% (mass ratio) and the rest is Pb]. Also, themelting temperature of lead-free solder is relatively high, 190 to 230°C., whereas that of tin-lead eutectic solder is 183° C. Therefore, whenlead-free solder is used, the stress generated by difference in thermalexpansions and thermal shrinkages between the multi-layer circuit boardand the case of the electronic device during the soldering process orthe like is increased and the stress reduction effect by solder isdecreased, and therefore the stress applied to the circuit board isincreased. For that reason, the occurrence rate of the phenomenon inwhich the circuit board is broken, in particular, at the through holeportion at the outermost end of the electronic device, is increased. Inother words, although such a phenomenon may occur when conventionaltin-lead eutectic solder is used, the occurrence of this phenomenon isincreased after the shift to lead-free solder.

More specific explanations are given of such a phenomenon, in which thecircuit board is broken, with reference to FIGS. 2, 3A to 3C. FIG. 2 isa cross-sectional view showing a state in which an electronic device issoldered on conventional multi-layer circuit board 1 shown in FIGS. 1Aand 1B by using lead-free solder. Also, FIGS. 3A to 3C are enlargedcross-sectional views showing the portion of outermost end through hole4 b in FIG. 2. These cross-sectional views are drawings based oncross-sectional photographs when an electronic device having a polyamidecase and provided with a connector having 8-pins arranged in one-line orleads is soldered with lead-free solder (Sn-3.0Ag-0.5Cu) on a circuitboard, which has FR-4 as a base material.

As shown in FIG. 2, each of leads 8 extending from case 7 of theelectronic device is inserted into each of through holes 4 and iselectrically and mechanically connected to a conductive film on theinternal wall surface of each of through holes 4 and each of lands 6with solder fillet 9. At this time, a lead of leads 8, which is insertedinto center through hole 4 a, is soldered so that its center almostcoincides with the center of center through hole 4 a and it extendsalmost vertically relative to multi-layer circuit board 1. On the otherhand, a lead of leads 8, which is inserted into outermost end throughhole 4 b, is soldered so that its tip is positioned away from the centerof outermost end through hole 4 b in a direction (outside direction)opposite to the side of centerline ◯ of case 7 of the electronic deviceat the time when it is mounted, and the lead is tilted in a directiontoward centerline ◯ of case 7 of the electronic device with progressiontoward the foot side (upper side in the drawing) of the correspondinglead of leads 8.

The reason that the electronic device is mounted like this is that case7 of the electronic device and multi-layer circuit board 1 are differentin materials and thus are different in thermal expansion coefficients,in particular, in the example shown in the drawings, the thermalexpansion coefficient of case 7 of the electronic device is larger thanthat of multi-layer circuit board 1. Before the soldering process isperformed, in outermost end through hole 4 b, the center thereof almostcoincides with the center of the corresponding lead of leads 8. However,during the soldering process, case 7 of the electronic device isthermally expanded by a larger amount than multi-layer circuit board 1,and therefore the relative position of case 7 of the electronic deviceand multi-layer circuit board 1 is shifted. The father is the positionfrom centerline ◯ of case 7 of the electronic device, the larger is theshift. Therefore, the outermost end lead of leads 8 is soldered on theposition that is considerably shifted in a direction opposite to adirection from the center of outermost end through hole 4 b to thecenter of case 7 at the time when the electronic device is mounted.Then, after the soldering process, as the temperature lowers, case 7 ofthe electronic device is thermally shrunk by a larger amount thanmulti-layer circuit board 1, and thus case 7 pulls the outermost endlead of leads 8 toward the side of the center of case 7 so that theoutermost end lead of leads 8 is tilted in the direction towardcenterline ◯ of case 7 with progression toward the upper side in thedrawings.

As shown in FIG. 3A, the outermost end lead of leads 8 is soldered atthe position that is shifted in the direction opposite to the directionfrom the center of outermost end through hole 4 b toward the center ofcase 7 at the time when the electronic device is mounted, and therefore,the amount of the solder in an area (A-portion: shaded area in thedrawing) in the direction opposite to the center direction of the caseof the electronic device, between through holes 4 and leads 8, isreduced. Then, due to temperature drop after the soldering process,leads 8 are tilted to the center of case 7 with progression toward theupper side in the drawings. At this time, because the amount of thesolder in A-portion is small as abovementioned and lead-free solder haslower stress reduction capacity than the tin-lead eutectic solder, theeffect that the lead-free solder absorbs and releases the stressgenerated by tilting of leads 8 is extremely small. Therefore, greatstress is applied to corner portion B and internal wall surface C in theportion in the direction opposite to the center of the case at the timewhen the electronic device is mounted. Accordingly, as shown in FIG. 3B,corner crack 11 is apt to occur at corner portion B of outermost endthrough hole 4 b, and, as shown in FIG. 3C, through hole separation 12,in which the conductive film covered on the internal wall surface ofoutermost end through hole 4 b peels, is apt to occur. In such asituation, faulty electrical continuity of the electronic device willoccur.

DISCLOSURE OF INVENTION

The present invention is made by taking the above problems intoconsideration, and thus has as an object to provide a highly reliablecircuit board that can reduce the occurrence of through hole cracks andthrough hole separations, even though an electronic insert mountingdevice is mounted with lead-free solder.

To achieve the above object, a circuit board of one aspect of thepresent invention has a plurality of through holes into which aplurality of leads of one electronic device are inserted and soldered.Then, among these through holes, the volume of a through hole, intowhich an outermost end lead of the leads of the electronic device isinserted, is larger than the volume of a through hole, into which a leadof the leads which is located at a position nearest to a center of theelectronic device is inserted.

In particular, in the circuit board of one aspect of the presentinvention, the plane shape of each through hole is a circle, and adiameter of the through hole, into which the outermost end lead of theelectronic device is inserted, is larger than the diameter of thethrough hole, into which the lead at the position nearest to the centerof the electronic device is inserted.

Also, in a circuit board of another aspect of the present invention, theplane shape of the through hole, into which the lead at the positionnearest to the center of the electronic device is inserted, is a circle,while the plane shape of the through hole, into which the outermost endlead of said electronic device is inserted, is an ellipse. The majoraxis direction of the ellipse is parallel with a line that connects thecenter of the corresponding through hole and the center position of theelectronic device at the time when the electronic device is mounted.Then, the length of the major axis of the ellipse is longer than thediameter of the through hole, into which the lead at the positionnearest to the center of the electronic device is inserted.

Also, in a circuit board of a further aspect of the present invention,the size of the through hole, into which the outermost end lead of theelectronic device is inserted, the size being measured in a direction ofa straight line connecting a position of the outermost end lead of theelectronic device, which is mounted before being soldered, and a centerposition of the electronic device at the time when the electronic deviceis mounted, is larger than the size in any direction in a plane of thethrough hole into which the lead at the position nearest to the centerof the electronic device is inserted.

Also, in a circuit board of a further aspect of the present invention,when a thermal expansion coefficient of the electronic device is largerthan a thermal expansion coefficient of the circuit board, the centerposition of the through hole, into which the outermost end lead of theelectronic device is inserted, is shifted in a direction away from thecenter position of the electronic device at the time when the electronicdevice is mounted, from a position of the outermost end lead of theelectronic device, which is mounted before being soldered. On the otherhand, when the thermal expansion coefficient of the electronic device issmaller than the thermal expansion coefficient of the circuit board, thecenter position of the through hole, into which the outermost end leadof the electronic device is inserted, is shifted in a directionapproaching the center of the electronic device at the time when theelectronic device is mounted, from a position of the outermost end leadof the electronic device, which is mounted before being soldered.

According to the above-mentioned circuit board of each aspect of thepresent invention, when the position of the outermost end lead isshifted due to the difference of the amount of thermal expansion betweenthe case of the electronic device and the circuit board during thesoldering process, in the outermost end through hole into which theoutermost end lead of the electronic device is inserted, among throughholes to be soldered, it is ensured that a sufficient amount of solderexists between the lead and the internal wall surface in the throughhole at a position in the lead-shifting direction. Therefore, asufficient amount of solder can absorb the stress applied to the lead inthe course of cooling after the soldering process. Accordingly, thestress applied to the corner portion and the internal wall surface ofthe outermost end through hole can be reduced, the occurrence of cornercracks or through hole separations in the outermost end through hole canbe reduced, and reliability for ensuring electric continuity in mountingan electronic device can be improved. According to the presentinvention, in particular, when soldering is performed by using lead-freesolder, reliability for ensuring electric continuity can besignificantly improved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a plan view of an example of a conventional multi-layercircuit board.

FIG. 1B is a cross-sectional view taken along line A-A in FIG. 1A.

FIG. 2 is a cross-sectional view showing a state in which an electronicdevice is mounted on the multi-layer circuit board in FIG. 1A.

FIG. 3A is a cross-sectional view for explaining a problem in themulti-layer circuit board in FIG. 1A.

FIG. 3B is further cross-sectional view for explaining a problem in themulti-layer circuit board in FIG. 1A.

FIG. 3C is further cross-sectional view for explaining a problem in themulti-layer circuit board in FIG. 1A.

FIG. 4A is a plan view of a multi-layer circuit board according to afirst embodiment of the present invention.

FIG. 4B is a cross-sectional view taken along line A-A in FIG. 4A.

FIG. 5 is a cross-sectional view showing a state in which an electronicdevice is mounted on the multi-layer circuit board in FIG. 4A.

FIG. 6A is a plan view of a multi-layer circuit board according to asecond embodiment of the present invention.

FIG. 6B is a cross-sectional view taken along line A-A in FIG. 6A.

FIG. 7 is a plan view of a multi-layer circuit board according to athird embodiment of the present invention.

FIG. 8 is a plan view of a multi-layer circuit board according to afourth embodiment of the present invention.

FIG. 9 is a plan view of a multi-layer circuit board according to afifth embodiment of the present invention.

FIG. 10 is a plan view of a multi-layer circuit board according to asixth embodiment of the present invention.

FIG. 11 is a plan view of a multi-layer circuit board according to aseventh embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION First Embodiment

FIG. 4A shows a plan view of multi-layer circuit board 11 according tothe first embodiment of the present invention, viewed from the side ofan electronic device mounting surface, and FIG. 4B shows across-sectional view taken along line A-A in FIG. 4A. Although manyelectronic surface mount devices and electronic insert mounting devicesare mounted on multi-layer circuit board 11, FIG. 1 shows only a portionthat are located at the mounting position of one electronic insertmounting device. Specifically, in the present embodiment, the electronicinsert mounting device has a plurality of leads 18 arranged in a line(see FIG. 5). Likewise, the drawings for the other embodiments show onlya portion that are located at the mounting position of one electronicinsert mounting device.

As shown in FIGS. 4A and 4B, multi-layer circuit board 11 includes resinlaminate 12 having internal wire 13 inside, as a base element. Throughholes 14 are formed at the insertion positions of leads of theelectronic insert mounting device. Lands 16 are formed at the edges ofopenings of through holes 14 on both sides of the board. External wires15 are also formed on both sides of the board.

In FIGS. 4A and 4B, a dashed-dotted line indicates centerline ◯ of case17 of the electronic device in the alignment direction of leads 18 (seeFIG. 5) at the time when the electronic device is mounted. In thepresent specification, among through holes 14, one that is at theposition nearest to centerline ◯ is described as center through hole 14a, and one that is at the farthest position is described as outermostend through hole 14 b. In the present embodiment, diameter Db ofoutermost end through hole 14 b is larger than diameter Da of otherthrough holes 14 including center through hole 14 a.

When the thermal expansion coefficient of the case of the electronicdevice is α(ppm/° C.) and the thermal expansion coefficient ofmulti-layer circuit board 11 is β(ppm/° C.), the relationship of α>β, issatisfied when the material of the case of the electronic device isplastic. In this case, diameter Db of outermost end through hole 14 b issuitably set as:Db={(α−β)×L}×2×ΔT+Da>Dawhere the distance between the center of case 17 of the electronicdevice and the center of outermost end through hole 14 b is indicatedwith L, and where the temperature difference of the case of theelectronic device before and after the soldering process is indicatedwith ΔT, namely, ΔT={[maximum temperature (approximate 200° C.) of thecase of the electronic device during the soldering process]−[roomtemperature (approximate 25° C.)]}. In other words, according to thissetting, even if the position of the outermost end lead of leads 18 ofthe electronic device is shifted by being heated during the solderingprocess, a certain distance can be kept between the corresponding leadof leads 18 and the wall surface at the side opposite to the center ofcase 17 of the electronic device and a sufficient amount of solder canbe kept between this wall surface and the corresponding lead of leads18.

Further, Db/2>P/2 must be satisfied in accordance with a bearing on thespace between outermost end through hole 14 b and a through hole ofthrough holes 14 which is adjacent thereto, in particular, in accordancewith a bearing on pitch P due to the fact that intervals of respectivethrough holes 14 are set constant in the example shown in the drawings.

Generally, pitch P between adjacent through holes 14 is often set tosatisfy the relationship of P>2Da, and therefore diameter Db ofoutermost end through hole 14 b is desirably set within the followingrange:Da<Db<2Da

Further, when the diameter of the land for outmost through hole 14 b isindicated with D and when D<2Da is satisfied, the following relationshipis satisfied:Da<Db<D.

Needless to say, consideration has to be given to board strength and soon, when the diameter of outermost end through hole 14 b is determined.

Using multi-layer circuit board 1, which is structured like this, a flowprocess is performed to mount the electronic device. For example, theflow process is performed as follows: 1. mounting an electronic deviceon multi-layer circuit board 11 at a predetermined position; 2. applyingflux to a side of multi-layer circuit board 11 where it will come intocontact with the solder jet; 3. preheating; 4. immersing multi-layercircuit board 11 in a solder bath that sprays jets of melted lead-freesolder; and 5. cooling.

The main process condition in the flow process is set so that lead-freesolder can reach lands 16 at the side of a surface, on which the deviceis mounted, to form solder fillets, for example, as follows:

Preheating temperature: 100 to 120° C.

Conveyor speed: 0.8 to 1.2 m/min

Solder jet: double wave

Solder bath temperature: 250±5° C.

FIG. 5 is a cross-sectional view showing a state in which the electronicdevice is mounted on multi-layer circuit board 11 according to the firstembodiment shown in FIG. 4. As shown in FIG. 5, the electronic devicehaving case 17 and leads 18 is mounted on multi-layer circuit board 11,and solder fillets 19 are formed in through holes 14 and on lands 16. Asdescribed above, in the present embodiment, among through holes 14,diameter Db of outermost end through hole 14 b, into which an outermostend lead of leads 18 of the electronic device is inserted, is largerthan diameter Da of center through hole 14 a, into which a lead of leads18 which is located at the position nearest to the center of theelectronic device is inserted (i.e., Db>Da). Therefore, the amount ofthe solder filled in outermost end through hole 14 b is larger than theamount of the solder filled in center through hole 14 a. In particular,in outermost end through hole 14 b, a sufficient amount of the soldercan be kept in the area at the side opposite to centerline O of case 17of the electronic device relative to the corresponding lead of leads 18,i.e., A-portion shaded in FIG. 5. Accordingly, the stress which iscaused because the corresponding lead of leads 18 of the electronicdevice is tilted in the course of cooling after the soldering process,can be reduced by the sufficient amount of lead-free solder. Therefore,the stress applied to the wall surface and the corner portion at theside opposite to the center of the electronic device can be reduced, andthe occurrence of corner cracks and through hole separations can bereduced.

Second Embodiment

FIG. 6A shows a plan view of multi-layer circuit board 21 according tothe second embodiment of the present invention, viewed from the side ofthe surface on which the device is mounted, and FIG. 6B shows across-sectional view taken along line A-A in FIG. 6A. In FIGS. 6A and6B, the corresponding elements are denoted by the same references as thefirst embodiment in FIGS. 4A and 4B, and repeated explanations areomitted.

The second embodiment is different from the first embodiment in regardto the point that the shape of outermost end through hole 24 b is anellipse. The major axis direction of this ellipse is the directionparallel to a line that connects the center of center through hole 14 aand outermost end through hole 24 b at the time when the electronicdevice is mounted, that is, the longitudinal direction of the case (thedirection indicated by arrow B in drawings) in the present embodiment.Major axis Dc of outermost end through hole 24 b is larger than diameterDa of center through hole 14 a, i.e., Dc>Da is satisfied. In regard to ahole that will become outermost end through hole 24 b, such anellipse-shaped hole may be formed by drilling while moving the drill orthe board that will become multi-layer circuit board 21. The length ofthe minor axis of outermost end through hole 24 b may be equal to thediameter of center through hole 14 a.

When an electronic device that is similar to the first embodiment andthat is shown in FIG. 5, is mounted on multi-layer circuit board 21according to the present embodiment, the amount of solder filled inoutermost end through hole 24 is larger than the amount of solder filledin center through hole 14 a. In particular, in outermost end throughhole 24 b, a sufficient amount of solder can be kept in the area (thearea corresponding to A portion in FIG. 5) at the side opposite to thecenter side of case 17 of the electronic device relative to thecorresponding lead of leads 18 of the electronic device, and thereforethe same effects as the first embodiment can be obtained.

Third Embodiment

FIG. 7 shows a plan view of multi-layer circuit board 31 according tothe third embodiment of the present invention, viewed from the side ofthe surface on which the device is mounted. The present embodiment isdifferent from the first embodiment shown in FIGS. 4A and 4B with regardto the point that not only outer most end through hole 34 b but alsoother through holes 14 vary in size in the present embodiment, asopposed to the first embodiment in which diameters of through holes 14,other than outermost end through hole 14 b, are equal. Specifically, inthe present embodiment, the diameter of center through hole 14 a is thesmallest size, and the diameter of the through hole, which is fartherfrom center through hole 14 a and closer to outermost end through hole34 b, is larger in stages.

Therefore, according to the present embodiment, in regard to throughholes 14 between outermost end through hole 34 b and center through hole14 a, the stress applied to the wall surfaces and the corner portionsthereof can be reduced, and thus mounting reliability can be furtherimproved.

Fourth Embodiment

FIG. 8 shows a plan view of multi-layer circuit board 41 according tothe fourth embodiment of the present invention, viewed from the side ofthe device mounting surface. The present embodiment is different fromthe first embodiment shown in FIGS. 4A and 4B with regard to the pointthat the plane shape of through hole 44 is a rectangle in the presentembodiment. In the present embodiment, all through holes other thanoutermost end through hole 44 b are similar to center through hole 44 ain size and in shape, however, the long side and the short side ofoutermost end through hole 44 b are longer than the long side and theshort side of center through hole 44 a, respectively.

Fifth Embodiment

FIG. 9 shows a plan view of multi-layer circuit board 51 according tothe fifth embodiment of the present invention, viewed from the side ofthe device mounting surface. While the first to fourth embodiments showexamples in which the electronic device has leads 18 arranged in a line,leads of an electronic device to be mounted on multi-layer circuit board51 according to the present embodiment are arranged at the center of thecase and at a plurality of positions that are located at equal distancesfrom the center in radius directions. In the present embodiment, centerthrough hole 14 a formed at a position nearest to center point ◯ of thecase of the electronic device at the time when the electronic device ismounted, and in the present embodiment, formed at the positioncoincident with the center point, is shaped into a circle of Da indiameter. On the other hand, outermost end through holes 54 b, i.e.,through holes other than center through hole 14 a, which are arranged atthe plurality of positions at equal distances from center point ◯, areformed in ellipse shapes. The major axis direction of the ellipse is thedirection of the line that connects the center point of thecorresponding through hole and center point O of the case of theelectronic device. Also, the length of major axis Dc of outermost endthrough holes 54 b are larger than diameter Da of center through hole 14a.

Sixth Embodiment

FIG. 10 shows a plan view of multi-layer circuit board 61 according tothe sixth embodiment of the present invention, viewed from the side ofthe device mounting surface. While the first to fourth embodiments showexamples in which the electronic device has leads 18 arranged in a line,the present embodiment shows an example in which an electronic device,that has leads arranged in two lines, is mounted. In the presentembodiment, when the electronic device is mounted, center through hole14 a formed at the position nearest to point O at the center point ofthe case portion is formed into a circle, however, outermost end throughhole 64 into which the outermost end lead is inserted is formed into arectangle whose two opposite sides are formed in hemicycles. A hole insuch a shape that will become outermost end through hole 64 b, may beformed by drilling while moving the drill or the board that will becomemulti-layer circuit board 61. The longitudinal direction of outermostend through hole 64 b, i.e., the direction in which two parallel sidesextend, is parallel with straight line L that connects point O where thecenter point of the case is positioned when the electronic device ismounted, and point P where the outermost end lead of the electronicdevice, which is mounted before being soldered, i.e., before beingheated, is positioned.

Seventh Embodiment

FIG. 11 shows a plan view of multi-layer circuit board 71 according tothe seventh embodiment of the present invention, viewed from the side ofthe surface on which the device is mounted. The present embodiment isdifferent from the sixth embodiment shown in FIG. 10 with regard to thepoint that outermost end through hole 74 b has a circular plane shape,and the center position thereof is shifted from point P where theoutermost end lead of the electronic device, which is mounted beforebeing soldered, i.e., before being heated, is positioned, in thedirection away from center O of the case of the electronic device. Inthe present embodiment, the center position of outermost end throughhole 74 b is located on the extension of straight line L that connectspoint P and point O. Further, outermost end through hole 74 b is largerthan center through hole 14 a.

As described above, since the center position of outermost end throughhole 74 b is sifted to the outside, a sufficient amount of solder can bekept in the portion (the area corresponding to A portion in FIG. 5) atthe side away from center O of the case relative to the lead inoutermost end through hole 74 b when the electronic device is mounted.According to the present embodiment, compared with the case in which thecenter position of the outermost end through hole is not shifted, evenif the diameter of outermost end through hole 74 b is made smaller (evenif the ratio of the diameter of outermost end through hole 74 b to thediameter of center through hole 14 a is made smaller), the same effectsas in the case in which the center position of the outermost end throughhole is not shifted and the diameter is made larger, can be obtained. Inother words, according to the present embodiment, a hole that willbecome outermost end through hole 74 b can be made relatively small.Further, in some cases, the diameter of outermost end through hole 74 bmay be equal to the diameter of center through hole 14 a. Therefore,according to the present invention, lowering the mechanical strength andthe imposition of restrictions on the land area due to an increase ofthe plane shape of the outermost end through hole, can be prevented.

Shifting the center position of the outermost end through hole away fromthe center of the case of the electronic device can be done when thethermal expansion coefficient of the case of the electronic device islarger than the thermal expansion coefficient of the circuit board. Onthe other hand, when the thermal expansion coefficient of the case ofthe electronic device is smaller than the thermal expansion coefficientof the circuit board, the center position of the outermost end throughhole is suitably shifted to be close to the center direction of the caseof the electronic device.

Up to this point, the preferred embodiments have been explained,however, the present invention is not limited to these embodiments, anda modification within the scope of the present invention may beappropriately made. For example, in each embodiment, the multi-layercircuit board is explained as an example, however, the present inventionis also available to a double-sided circuit board. Also, the shape ofthe through hole may be a shape other than those explained in eachembodiment, and, for example, a gourd shape formed by drilling twice maybe available.

1. A circuit board having mounted thereon an electronic device having aplurality of leads and a case, wherein said circuit board has aplurality of through holes into which said plurality of leads of anelectronic device are inserted and soldered with lead-free solder:wherein a conductive film is formed on a wall surface of said throughholes, and a volume of a through hole of said through holes, into whichan outermost end lead of said leads of said electronic device isinserted, is larger than a volume of a through hole of said throughholes, into which a lead of said leads, which is located at a positionnearest to a center of said electronic device, is inserted, and whereinthe distal ends of said leads are tilted towards a center of the case ofthe electronic device.
 2. The circuit board according to claim 1,wherein a plane shape of each of said through holes is a circle, andwherein a diameter of said through hole, into which said outermost endlead of said electronic device is inserted, is larger than a diameter ofsaid through hole, into which said lead at the position nearest to thecenter of said electronic device is inserted.
 3. The circuit boardaccording to claim 2, wherein the diameter of said through hole, intowhich said outermost end lead of said electronic device is inserted, isnot more than twice the diameter of said through hole, into which saidlead at the position nearest to the center of said electronic device isinserted.
 4. The circuit board according to claim 1, wherein shapes ofthrough holes of said through holes, which are located between saidthrough hole, into which said lead at the position nearest to the centerof said electronic device is inserted, and said through hole, into whichsaid outermost end lead of said electronic device is inserted, aregradually changed from a shape of said through hole into which said leadat the position nearest to the center of said electronic device isinserted to a shape of said through hole into which said outermost endlead of said electronic device is inserted.
 5. The circuit boardaccording to claim 1, wherein a center position of said through holeinto which said outermost end lead of said electronic device is insertedis shifted in a direction away from a center position of said electronicdevice at the time of being mounted, from a position of said outermostend lead of said electronic device, which is mounted before beingsoldered, when a thermal expansion coefficient of said electronic deviceis larger than a thermal expansion coefficient of said circuit board,and the center position is shifted in a direction approaching a centerof said electronic device at the time of being mounted, from a positionof said outermost end lead of said electronic device, which is mountedbefore being soldered, when the thermal expansion coefficient of saidelectronic device is smaller than the thermal expansion coefficient ofsaid circuit board.
 6. A circuit board having mounted thereon anelectronic device having a plurality of leads and a case, wherein saidcircuit board has a plurality of through holes into which said pluralityof leads of an electronic device are inserted and soldered withlead-free solder: wherein a conductive film is formed on a wall surfaceof said through holes, and a plane shape of a through hole of saidthrough holes, into which a lead of said leads which is located at aposition nearest to a center of said electronic device is inserted, is acircle, wherein a plane shape of a through hole of said through holes,into which an outermost end lead of said leads of said electronic deviceis inserted, is an ellipse having a major axis in a direction parallelwith a line that connects a center of the corresponding through hole anda center position of said electronic device at a time of being mounted,and wherein a length of the major axis of said ellipse is longer than adiameter of said through hole, into which said lead at the positionnearest to the center of said electronic device is inserted, wherein thediameter of said through hole, into which said lead at the positionnearest to the center of said electronic device is inserted, is at leastas long as a minor axis of said ellipse, and wherein the distal ends ofsaid leads are tilted towards a center of the case of the electronicdevice.
 7. A circuit board having mounted thereon an electronic devicehaving a plurality of leads and a case, wherein said circuit board has aplurality of through holes into which said plurality of leads of anelectronic device are inserted and soldered with lead-free solder:wherein a conductive film is formed on a wall surface of said throughholes, and a size of a through hole of said through holes, into which anoutermost end lead of said leads of said electronic device is inserted,the size being measured in a direction of a line that connects aposition of said outermost end lead of said electronic device beingmounted before being soldered and a center position of said electronicdevice at a time of being mounted, is larger than a size of a throughhole of said through holes, into which a lead of said leads which islocated at the position nearest to the center of said electronic deviceis inserted, the size being measured in any direction in a plane, andwherein the distal ends of said leads are tilted towards a center of thecase of the electronic device.
 8. The circuit board according to claim7, wherein an opening of said through hole into which said outermost endlead of said electronic device is inserted is formed by drilling morethan once or by moving a drill relative to the board.
 9. A circuit boardhaving mounted thereon an electronic device having a plurality of leadsand a case, wherein said circuit board has a plurality of through holesinto which said plurality of leads of an electronic device are insertedand soldered with lead-free solder: wherein a conductive film is formedon a wall surface of said through holes, and a center position of athrough hole of said through holes, into which an outermost end lead ofsaid leads of said electronic device is inserted, is shifted in adirection away from a center position of said electronic device at thetime of being mounted, from a position of said outermost end lead ofsaid electronic device, which is mounted before being soldered, when athermal expansion coefficient of said electronic device is larger than athermal expansion coefficient of said circuit board, and the centerposition is shifted in a direction approaching a center of saidelectronic device at a time of being mounted, from the position of saidoutermost end lead of said electronic device, which is mounted beforebeing soldered, when the thermal expansion coefficient of saidelectronic device is smaller than the thermal expansion coefficient ofsaid circuit board, and wherein the distal ends of said leads are tiltedtowards a center of the case of the electronic device.